http://www.pokemon.com/pokemonxy/en-gb - It's assumed that it's a new Pokemon because of the image on the official Pokemon X&Y website. It says "who is this new Pokemon?", so yea, the main assumption is that it's not a Mewtwo form, but an entirely different Pokemon.

In any case, it's right to say that we won't know for sure until it's officially confirmed.

i did, that's exactly what I mean...it looks similar, so it should be "related" (i.e. manaphy & phione) but if its the same typing and has a very similar body type, it really shouldn't be another form. I wouldn't see the point of changing in between them (if I was that pokemon).

Its a form of mewtwo, a mega form which happens when you mega evolve it by giving it its mega stone and activating it during battle, there are two forms mega mewtwo x and mega mewtwo y,
both bare exclusive to their game